1. Field of the Invention
The present invention relates to a development device, an image forming apparatus and a development method.
2. Discussion of the Background
In general, an image forming apparatus such as a photocopier, a printer, a facsimile machine or a multi-function apparatus thereof has a configuration in which paper accommodated in a paper cassette is transferred to a paper path where an image is printed on the paper and discharged to a discharging tray.
The development devices for use in an image forming apparatus such as an electrophotographic photocopier or an electrophotographic printer are typified into a two component development device using a development agent including a carrier and a toner and a single component development device using a development agent including only a toner. Demands with regard to the image forming apparatus from the market are, for example, size reduction, low manufacturing cost, improvement on the quality of images, high speed performance, etc. A single component development agent is preferably used.
A single component development device is well known which includes a development roller which is arranged adjacent to a latent electrostatic image bearing member and bears a toner as a single component development agent, a development agent supply roller to supply toner to the development roller, and a regulation applicator which is brought in contact with the surface of the development roller to regulate the layer thickness of the toner on the development roller (refer to unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2001-255737).
A magnetic single component development agent (toner) and a non-magnetic single component development agent (toner) are used as the single component development agent for use in the single component development device. As described in JOP S58-223158, since the magnetic single component development agent is easily attached to the surface of the development roller by a magnetic attraction force, there is a wide latitude with regard to the supply direction of the toner and also a wide designing latitude with regard to the designing of the development device itself and the arrangement thereof in a photocopier. However, since a magnetite prescribed as a magnetic component in the magnetic single component development agent has its own inherent color of blue or brown, toner for black can be manufactured for photocopying but no toner for cyan, magenta or yellow can be suitably manufactured. In addition, the mixed color obtained at image fixing is not perfectly the original color because the inherent color of magnetite is reflected at color mixture.
On the other hand, the non-magnetic single component development agent is only formed of resin components, which is advantageous to manufacture pure cyan, magenta, or yellow color toner for color images. However, since the toner is not attracted to the development roller by a magnetic attraction force, the toner is supplied to the surface of the development roller by making the toner in contact with the development roller under a significantly constant pressure. That is, the toner layer is formed on the surface of the roller by interaction such as Coulomb's force or van del Waals's force based on triboelectrical charge. Therefore, the supply direction of toner to the development roller is limited in some degree and the designing latitude is narrow.
That is, the toner in the non-magnetic single component development system is necessary to be regulated under a significantly constant pressure applied by, for example, a metal blade. This causes a problem that while a development device is used for an extended period of time, some of the toner is adhered to the blade, which easily prevents the toner from forming a uniform thin layer on the surface of the roller. To be specific, streaks are formed on the thin layer of the toner on the development roller along the rotation direction thereof and reflected on an image. This means that the non-magnetic single component development method involves with the image degradation problem caused by adhesion of toner to the regulation applicator such as a blade.
The toner adhesion to a regulation applicator is caused, for example, in such a manner that fine powder toner having a smaller particle diameter than the average particle diameter of the toner accumulates on the regulation applicator (blade) and adheres thereto in due time while the fine powder toner is abraded by the regulation applicator. The fine powder toner is produced when toner which has passed through the regulation applicator is under a great stress such that the toner is abraded between the regulation applicator and the development roller and resultantly cracks and/or is chipped off. The toner particles have a particle diameter with a distribution in a particular range and fine powder toner having a small particle diameter is known to have a relatively small amount of charge in comparison with that of toner having an average particle diameter. The fine powder toner having a small amount of charge which is produced when the toner passes the regulation applicator tends not to be used for development of an image on an image bearing member but to remain on the surface of the development roller. The fine powder toner remaining on the surface of the development roller is collected at (transferred to) the toner supply roller portion, accumulated around the toner supply roller and supplied again to the regulation applicator. The fine powder toner produced by cracking or chipping-off contains wax, a pigment, a dispersion agent, a charge control agent in a large amount. It is known that as the amount (ratio) of the fine powder toner increases, the fluidity of the toner decreases. The fine powder toner, which has a small amount of charge and a low fluidity, tends to accumulate at the nip portion where the toner is nipped between the development roller and the regulation applicator for an extended period of time, which is the main cause of the adhesion of the toner to the regulation applicator.
The non-magnetic single component development agent has another problem, which is the non-magnetic single component development agent is directly adhered to the regulation applicator under a great amount of pressure thereby. However, for example, JOP 2001-255737 describes solutions to this problem, which are controlling the regulation pressure to toner by a regulation applicator to avoid a great pressure, applying a bias having the same polarity as that of toner to prevent the toner from adhering to the regulation applicator (blade), etc.
As a method of preventing the adhesion of fine powder toner to the regulation applicator, JOP H07-168393 describes a method in which fine powder toner having a particle diameter of 5 μm or smaller is controlled to be 2% or less by volume distribution when manufacturing toner.
In addition, JOP 2005-189405 describes a configuration in which a rotation brush is arranged in contact with the development roller to generate an electric field. When a rotation brush having a low implant density is used, all the toner including fine powder toner in the development device may be captured. Therefore, a rotation brush having a high implant density is preferably used.
However, in recent years, development of a compact-sized apparatus capable of producing images at a high speed has been demanded. To meet this demand, the pressure applied to toner by a regulation applicator increases so that the adhesion problem of toner to the regulation applicator still remains unsolved because fine powder toner is newly produced under the pressure by a regulation applicator after fine powder toner produced in the process of manufacturing toner is removed as described in JOP H07-168393. The fine powder toner produced in the development device has a large impact in comparison with the fine powder toner contained at the manufacturing stage. Especially the fine powder toner has an adverse impact in an environment of high temperature and a high humidity.
As described above, the fine powder toner newly produced in a development device is produced when the toner cracks under a great pressure from a regulation applicator. External additives are not attached to the new surface of the cracked toner. Isolated external additives in toner can be attached to the new surface but does not impart the fine powder toner with a sufficient attachment property. Therefore, the fine powder toner newly produced in a development device has a low chargeability in comparison with fine powder toner produced when manufacturing toner and a strong attachment property to a regulation applicator when compared with a toner to which external additives are regularly attached.
In addition, the method described in JOP 2005-189405 decreases the amount of the fine powder toner by the rotation brush but not to a sufficient level.